Structural sandwich panel deck



Jan. 18, 1966 w. H. MILES 3,229,433

STRUCTURAL SANDWICH PANEL DECK Filed Aug. 28, 1962 A l) f5 5 H f2 f f Nf f :WwW/7 Y?? Y? 2 United States Patent O 3,229,433 STRUCTURAL SANDWICHFANEL DECK William H. Miles, Tracyton, Wash., assigner to the UnitedStates of America as represented by the Secretary of the Navy Filed Aug.28, 1962, Ser. No. 229,097 6 Claims. (Ci. 52-309) (Granted under Title35, U.S. Code (1952), sec. 266) The invention described herein may bemanufactured and used by or for the Government of the United States ofAmerica for governmental purposes without the payment of any royaltiesthereon or therefor.

The present invention relates to metal-faced plywood panelling and, inparticular, the invention features the use of such a paneling as adurable, impact-resistant oor or deck surface, such, for example, as theflight deck surface of an aircraft carrier.

Metal and wood composites have been rather widely used for a variety ofpurposes such, for example, as the wall facings for airplane cabins ordecorative and somewhat functional facings or claddings for furnitureand the like. These uses, of course, are radically different from anyutilization of similar composite materials as an impactresistant flooror deck surface.

As already indicated, the present invention particularly contemplatesusing a metal-wood composite as an aircraft carrier iiight deck which,of course, is subjected to extremes of impact, as well as otherdebilating forces such as friction, heat, moisture, as well as theability to accept heavy rolling loads and impacts without deflection andbreakage. This latter consideration is quite important in ight deckconstruction to the extent that any such decking must provide adequateload distribution so as to avoid any deection or breakage in areas wherethe impacts are concentrated.

Other problems, particularly in ight deck construction, involve weight,strength and economic considerations. For example, design considerationsconstantly involve a compromise between strength and weight, the optimumbeing to provide a light weight strong decking of low initial,replacement and maintenance cost, and having sufficient strength toaccommodate modern jet aircraft with their high speed landings andtake-offs. In the past, it has been customary to form such decks ofnarrow teakwood planking, although such planking presented a number ofdifculties including relatively poor abrasion resistance, a waterabsorption capability and excessive maintenance and replacement costs.More modern aircraft carriers utilize metal flight decks which are adistinct improvement but which, as it is known to those familiar withsuch construction, must be specially designed and built. In other words,it is not feasible to replace the older teakwood decks with the allmetal construction.

It is therefore an object of the present invention to provide a strong,durable, impact-resistant composite laminate door or deck surfaceparticularly suited for use as a flight deck for aircraft carriers.

A more specific object is to provide a composite laminate havingsubstantial impact resistance provided by a metal facing, the laminatebeing an adhesively bonded structure formed of separate wood and metalcourses with no need for through joints or other mechanical fastenings.

A further object is to provide an integrally-bonded, metal-facedsandwich construction formed of relatively Patented dan. 18,

large panels capable of receiving and adequately distributing loads andimpacts.

An object related in part to the last-mentioned object is to provide asuitable bond for the metal and wood of the laminate to the extent thatsubstantial elongations and contractions of the metal panels whensubjected to temperature variations is fully accommodated withoutrupture by the bonding material.

A further object which will become more apparent is to provide such acomposite integrally bonded metal faced construction for flight decksand the like, the construction permitting the lay-up of the materials insitu over the ight deck surface.

A further object related to the last object is to permit such a lay-upwithout the need for high temperature or pressure cures for the bondingmaterials.

A more generalized object is to provide a suitable metal faced sandwichconstruction adapted to be substituted for teakwood iiight decks to theextent that the composite can be directly applied to existing teakwoodight sub-surfaces.

Other objects and their attendant advantages will become more apparentin the ensuing description.

According to the invention the laminate structure is formed of a plywoodcore the bottom surface of which is bonded to the decks sub-surface orto other available supporting structure, while the top or upper surfaceis bonded in a special manner to a metal facing which provides theexterior impact resistant surface. The plywood core has multiple courseseach bonded one to the other by a suitable thin layer of adhesive andthese courses each are formed of panels of substantial individual size,such for example as 4 x 10 hickory or Douglas tir plywood panels of athickness of about l". Also, the invention features a staggered lay-upof the super-imposed courses of the panels and most, most suitably, thepanels of an upper layer are staggered one-half the width and length ofan underlying course or layer to provide core continuity. Similarly, themetal facing for the plywood core also is formed of panels preferably ofaluminum or steel, these metal panels having dimensions at least aslarge as the plywood panels and having a substantial thickness of atleast 1/s. In the preferred form, the metal panels are hooked around theends of underlying plywood panels to achieve a mechanicallyinterconnected arrangement insuring that the edges cannot be caught byaircraft tailhooks or the like and further insuring a firminter-engagement in the event adhesive bonding should fail at theseedges. This hooking or locking arrangement of the metal facing will bedescribed in further detail.

To provide an integral structure, all of the courses including the metalover-lay are adhesivelyl bonded one to the other and the laminate alsois adhesively bonded to its sub-structure or supporting surface. Mostsuitably, the bonding to the sub-structure is provided by a specialadhesive in the form of -a liquid synthetic rubber into which is mixed,preferably in equal parts, a quantity of phenolic microspheres whichprovide bulk to lill gaps usually found in deck sub-structures. Also, itis preferred to use a curable liquid synthetic rubber mastic for bondingthe plywood layers and a similar mastic is used between the metal facing-and the upper surface of the laminated plywood core.

An important feature of the invention lies in the fact that therubber-like adhesive between the metal and the plywood is formed ofsufficient thickness to iiexibly accommodate lateral and longitudinalexpansion and contraction of the met-al panels. The entire arrangementprovides adequate and durable strength and impact resistance dueparticularly to the panel construction both of the staggered plywoodcourses and the metal overlay or facing. Also, the integral bondingprovides a secure structure with-adequate watertight integrity and thesecurity of the structure is substantially augmented by the use of therubber-like mastic to provide a llexible bonding action between themetal and the plywood capable of accommodating thermal expansions. Inthis regard, it may be interesting to note that the relatively largepanel construction permits a distribution of impact and rolling forcesover a relatively wide area. By the same token, however, the use oflarge metal panels or plates of suflicient thickness to withstand impactpresents a thermal expansion and contraction problem which is met by theemployment of the flexible adhesive, although it is considered advisableto limit the length of the metal rpanels sufiiciently to minimizeexpansion and contraction and thereby permit a reasonable thickness ofmastic to be used between the metal and the plywood. Other features andadvantages of the arrangement will become more apparent and the detaileddescription which is to follow.

The preferred embodiment of the invention is illustrated in theaccompanying drawings of which FIG. 1 is a vertical section illustratingone embodiment of the composite laminate supported by and bonded toconventional steel pan flight deck sub-structure; FIG. 2 is aperspective n plan illustrating a staggered arrangement of the coursesmaking up the plywood core of the composite laminate; FIG. 3 is a viewsimilar to FIG. 1 illustrating another embodiment of the invention, andFIG. 4 is `a longitudinal vertical section through one panel of the FIG.3 embodiment, this view illustrating particularly the manner in whichthe metal panel is locked to a wood panel to form a top course of thecomposite laminate.

Referring to the drawings and, in particular, to the embodimentillustrated in FIG. 1, it will be seen that the composite laminate as aunit is formed of a plywood core 1 made up of super-imposed courses 2and 3, this core in turn being supported by and adhesively bonded to aflight deck sub-stru-cture such as steel pan 4. Finally, the laminate iscompleted by adhesively bonding a metal facing 6 to top course 2 of theplywood core.

In greater particular, plywood core 1 is formed in the manner best shownin FIG. 2 of two courses of plywood panels 7 which, preferably, areformed of Douglas iir, although other core materials such as highstrength, lightweight, impregnated hardboards may be substituted asconsidered desirable. Each of the panels is of substantial size, thepanels used in actual construction of the deck being 1A" X 4 X l0'. Asfar as dimensional considerations of the plywood panels are concerned,the important factor is to utilize panels of substantial size so thatweight or impact loads can be distributed over a relatively wide area.The size of the wood panels also will be dependent to some extent onconsiderations affecting the metal facing. The staggered arrangementillustrated in FIG. 2 is particularly important not only to provide corecontinuity with no through seams but also to increase the stiffness orrigidity of the core and permit greater impact resistance and loaddistribution. As shown, the two courses of the core preferably arestaggered one half the width and length of the panels.

Metal facing 6 also is formed of individual panels laid end to end and,as seen in FIG. 1, the ends of these panels are spaced one from theother approximately 1/s to provide an expansion gap 8 which, as shown,`are spaced from seams 9 of the wood panels to eliminate through seams.In the actual construction or lay-up of the composite on a ight deck, itwill be found that flight decks customarily include *ie-down iittings,studs and other similar protrusions so that the panels usually will haveto be patterned to clear such objects.

The laminate structure .as a whole is supported by the steel subdeck ofthe carrier, this deck usually being in the illustrated form having itssteel pan 4 supported at intervals by I-beams 11. It is particularlypertinent to note that the stretches of the pan between the I-beams sagsomewhat so that there are substantial thickness variations between thebottom of the plywood core and the steel pan.

Another feature of the invention is that it is an integrally bondedsandwich construction without through joints, the bond being formedpreferably by particular ilexible adhesives which will be described. Themost important adhesive bond of the composite is that between metalfacing 6 .and plywood panels 7 of the top course of the plywood core,the significance of this bond being readily recognizable from theobvious fact that the thermal conductivity and coecient of expansion ofthe metal, particularly aluminum, vary considerably from that ofplywood. Obviously, upon expansion of the metal panels, rigid bondingmaterial or adhesives soon would rupture and break loose, especiallywhen the panels are of a substantial length sufficient to distribute theanticipated loads and heavy impacts of the landing and take-olf ofmodern aircraft.

Before describing the adhesive, it also is important to note that thepanels of metal facing 6 are of substantial thickness as opposed to thethin sheets of aluminum or foils of other metals frequently used asfacings for plywood when the combination or composite lamination is toprovide wall coverings or decorative facings, nine specifically, metalpanels 1, if formed of aluminum, should be approximately i, or, ifformed of steel or of other stiffer metal, the thickness obviously canbe reduced. However, the thickness regardless of the metal materialemployed for the facing must be suflcient not only to withstandIanticipated impacts but also to avoid buckling Such as had been foundto occur when thin aluminum sheet is employed.

As already indicated, one of the important manners in which the presentinvention accommodates thermal expansion and contraction as well asavoids buckling of the metal facing is by the use of a exible adhesivewhich is provided in a relatively thick layer 12` between metal facing 6and the plywood panels of top plywood course 2. An elastomeric adhesiveis considered essential and this adhesive may be selected fromcommercially available elastomeric water, oil and ozone resistantmastic, gap-filling adhesives such as polysulde, neoprene orpolyurethane biased compounds. As may be noted, these materials areliquid synthetic rubbers. A further essential restriction upon theselection o-f adhesive involves, of course, its capability of retainingadhesion in the presence of extreme differential expansion. Forinstance, a l2 long aluminum sheet, such as is preferred in the FIG. 1embodiment, can be bonded to the upper course of the plywood panels witha polysulde having an average thickness of 5&4, it having been foundthat such a thickness can accommodate expansion and contraction withoutbond failure when subjected to- F. temperature variations. In otherwords, such a thickness accommodates temperature variations of 100 F. oneither side of anticipated ambient temperatures. The end movement ofsuch an aluminum sheet approximates Lg7/16 and, of course, it is forthis reason that expansion gaps 8 are left between the ends of the metalpanels.

The polysulfide elastomer preferred in the present laminate iscompounded by several suppliers and it consists o-f liquid polysuldetogether with compounding ingredients which tare well known in the artas appropriate mechanisms for reinforcing, stabilizing, and controllingviscosity of ow. The liquid polysulde component is a. reaction productof organic dihalides and sodium polysulfide, slightly depolymerized torender it liquid. For example, the liquid polysuliide may be a reactionproduct of dichloroethyl formal (98% mole), trichloropropane (2% tno-le)and sodium -polysultide in amounts proper for reaction. This liquidpolysuliide is known las Thiokol LP-Z, LP-3, LP-32 or LP-33 dependingupon the degree of reaction or viscosity as produced by the ThiokolCorporation. These particular polymers are described in Zimmerman andLavines Handbook of Material Tradenames, Supplement I, published in1956. Thiokol is a trademark for various products produced -by theThiokol Corporation including the above-identicd types, these compoundsbeing polymers which, as is well known in the art, can be converted inplace at room temperature to tough rubbers without appreciableshrinkage. The compounds also are resistant to solvents, oils andgre'ases and they have good aging characteristics and maintainrubber-like properties over a wide range. Typical formulations of thepolysulide compound suitable for use with a hardener for curing into asolid flexible elastomer at room temperature are:

P.b.w. Thiokol (LP-2, LP-S, LP-32, LP-33) 100 Stearic acid .51

Pigment (carbon black, zinc, suliide, titanium dioxide 30-80 An'accelerator or hardener for this liquid may be as follows:

P.b.w.

Lead peroxide 4.5

Stearic acid .5-1

Dibutyl phthalate 4 6.0-7.0

For improved adhesion, the following mixture may be added to theaccelerator:

P.b.w. Phenolic resin 5-15 Polyvinyl alcohol 1-2 or 2-(R)SH{ZNO(R)-S-ZN-S(R)l-H2O. As may be noted, these products are essentially 100%reactive so that the converted compounds demonstrate no appreciableshrinkage with the result that the bond between the met-al and theplywood is evenly distributed in its overall strength.

As to the bonding together of the plywood layers as well as the bottomface of the plywood core to steel pan 4, the adhesives employed for thispurpose may be the same as those previously described, although the useof tiexible adhesives for these latter purposes is not too criticalsince the thermal expansion and contnaction problem exists only to minorextents. However, such elastomeric adhesives are preferred principallybecause of their low shrinkage, their resistance to solvents, oils,greases etc. and their flexibility to accommodate whatever sheer forcesmay be imposed upon them. As will be noted in FIG. 1, ya thin iilm 13 ofadhesive is employed between the courses of plywood panels 2 and 3,while a thick layer 14 of adhesive fills the space between the bottomsurface of the plywood core and the steel pan.

This latter layer 14 is a specially-formulated adhesive to the extentthat, preferably, it is formed of :a mixture of equal parts of lthepreviously-described elastomeric adhesive and a microsphere iiller. Theiiller, more specitically, is a batch of hollow spheres of approximately2-7 microns, these spheres usually being iilled with an inert gas andbeing employed was a bulking agent which substantially reduces the costof the adhesive and also provides a strong integral .bonding of thepanel core to the steel pan. Bonding eliminates any need for numerousfastening studs such as commonly were used in prior -teakwood planking.The particular microspheres ernployed are known as BIO-0930 phenolicmicroballoons, the term microb-alloons being a tradename applied tomicrospheres produced by Bakelite Corporation.

The lay-up of the composite laminate shown in FlG. l is accomplished ina particular manner on location or, in carrier work, on the deck. Incommencing the lay-up, the panels first are patterned, then spray-coatedon all surfaces with a specified primer which is a matter of choicedepending upon the particular elastomeric adhesive or other adhesives tobe employed. Such primers usually are recommended by the commercialproducer of the elastomer. First course 3 of the plywood core then isbonded to the steel sub-deck with the leveling, gap-filling polysuldemastic-microsphere mixture already described. First, the steel deckshould be cleaned with a suitable rust preventative compound and thenthe leveling coat troweled down and leveled, this layer usuallyaveraging 1/1 in thickness with a minimum of 1,66 over the longitudinalsor I-beams. The bottom and edges of irst course 3 also are coated withthe same mixture preferably using a s x 1/s notched trowel and, afterinstallation, theA panels are well tamped to eliminate air pockets,following which they may be weighted with steel pigs and sandbags to anaverage of approximately 10 lbs. per square foot and left overnight.

Next, the top face of the iirst course and bottom face of the secondcourse of the primed plywood are coated by troweling a JAG" thick layerover each and, again, this layer after installation, is tamped, Weightedand permitted to cure. In all of these operations, it will be recognizedthat the mixing of the liquid synthetic rubber with the accelerator canbe done on location and these compounds usually have a working life of 2to 4 hours at ambient temperatures so that batches can be easily spreadand individual panels laid up within the working time limits. Solventfor such cleaning as may be necessary can be equal parts of toluene andmethyl ethyl ketone (MEK) and it will be found advisable to use rubbergloves during mixing, handling and spreading.

Prior to applying the metal panel overlay, the top of the second courseshould be surface-planeet, to eliminate unevenness at joints, and thenprime coated. Also, the back surfaces of the metal overlay (steel oraluminum) should be prepared for bonding by drum sanding or similarexpedient, the prepared surfaces of both the metal and the wood beingcoated by troweling or otherwise applying the relatively thickpolysulide or other elastomeric adhesive. To insure expansion gapswashers temporarily may be inserted along the edges of abutting metalpanels and the installed steel or aluminum overlay then is sandbaggedand permitted to cure for a period of about twenty-four hours. Theunfilled joints between the panels are filled flush with the sameadhesive employed for the bond.

Trial installations of a composite laminate corresponding to thatpreviously described have been made and evaluated under rigorous ightdeck conditions and for a lengthy trial period with excellent results.Thus, in spite of temperature variations F. on either side of ambient,as well as extreme impacts and debilitating forces to which thecomposite was subjected, bonding retained its continuity and the metaloverlay maintained its smooth, unbuckled and unwavy characteristics.Also, the composite has excellent watertight integrity and adequatelyresists other chemical deterioration. These factors, coupled with therelative ease and inexpense of lay-up have proven most encouraging. Theability to provide an integrally bonded composite with no accompanyingneed for studs or repeated caulking assures not only low installationbut also low maintenance costs. Further, the composition is relativelylight weight which is an important factor particularly in aircraftcarrier structure.

The embodiment of the composite laminate illustrated in FIG. 3 diifersfrom the FIG. l embodiment only in the manner in which metal panels areapplied. Otherwise, the use of first and second courses of plywood panelto form a plwvood core may be the same as the FIG. l embodiment, as isthe manner of bonding the plywood core to steel pan 1 and the bonding ofthe first and second courses of the core one to the other.

The principal differences between FIG. 3 and FIG. l embodiments is thefact that FIG. 3 embodiment employs a third plywood course 16 alsoformed of plywood panels of substantial length, width and thickness,these panels, however, being mechanically interlocked to metal plates 17which, in the manner already described, are bonded to the top surface ofthe panels of this third course. However, because of the use of thirdcourse 16, it may be found desirable to reduce the thickness of all theply- Wood courses to the extent that the previous thicknesses of courses2 and 3 may be reduced from Mt to l" while third course 16 is of athickness of 1% The metal plates, particularly if aluminum is employed,again are 1A in thickness so that the overall thickness of the compositeis approximately 3 as compared to 2% for the overall thickness of theFIG. 1 embodiment.

The mechanical interlocking of metal plates 17 with the panels of thirdcourse 16 is achieved by forming the end surfaces of each of which hasan inwardly extending ange 18 (FIG. 4) designed to iit beneath the Woodpanels in the manner shown to provide the interlock. To accommodateflanges 18, each of the plywood panels of the top course is undercut toprovide grooves 19. Also, l spacings 21 and 22 are left between the endsof the wood panel and the hook portion of the metal panels for obviousexpansion purposes.

The construction of the FIG. 3 embodiment may be utilized generally,although it is particularly suitable for use on the actual impact areaof an aircraft landing zone. Also, because of the anticipated severeimpacts, it might be preferred to employ hickory plywood at least forthe top or third 3/1" plywood course, the hickory being used because ofits hardness and higher strength. The interlock is advantageous ininsuring that edges of the lmetal cannot be caught by aircrafttailhooks. Also its use provides somewhat of a guarantee in the eventthe adhesive fails at the edges.

The operation of the composite, as well as its advantages should bereadily apparent from the foregoing description. As compared to priorteakwood planking, the advantages of cost and strentgh appearunquestionable. Also, although modern aircraft flight decks presentlyfavor all steel or metal construction, the strength and certainly thecost of the present laminate compares favorably. As already noted, oneconsiderable advantage is the fact that a laminate of the foregoing typecan be readily substituted for existing teakwood or other planktypedecks, while the all steel construction is not adaptable. It furthershould be :again noted that the flight deck application of the presentlaminate has been emphasized principally as exemplary of the requiredcharacteristics. Other possible applications are in the ield of minesweeper decks, industrial oors, bridge decks, landing strips :andfloating and submerged platforms.

Other objects and many of the attendant advantages of this inventionwill be readily appreciated as the same becomes better understood byreference to the following detailed description when considered inconnection with the accompanying drawings.

What is claimed is:

1. A composite wear and impact-resistant laminate comprising a plywoodcore formed of plural courses of ad-hesively bonded plywood panels,superimposed in a. staggered arrangement and a composite metal andplywood facing overlying and adhesively bonded to said core, saidcomposite facing being formed of looselyinterlocked metal and plywoodpanels of substantially equal length and of a length `capable ofproducing approximately 3/16 end movement of the metal panel whensubjected a temperature change of F., a layer of adhesive materialintegrally bonding said plywood panels of said facing to said plywoodcore, and another layer of flexible adhesive material integrally bondingthe under face of each metal panel to its interlocked plywood panel,said layer of flexible adhesive being of suicient thickness foraccommodating said end movement without rupture said interlock beingprovided by end ilanges formed on each metal panel, said flanges beingspaced from the end portions of the interl-ocked plywood panels asuiicient distance for accommodating said expansion and said end fiangeseach having a portion disposed beneath said plywood facing.

2. A composite Wear and impact resistant laminate comprising a coreformed of plural courses of wood panels superimposed in a staggeredarrangement, a metal facing overlying said core and formed of semi-rigidmetal panels, a lm of adhesive material integrally bonding superimposedwood courses one to the other, a second layer of adhesive materialintegrally bonding the underlying facing of the core to :a supportstructure, and a third layer of lexibly adhesive material integrallybonding the overlying face of the core to said metal facing, said coreand metal panels being substantially equal in length and said metalpanels being off suiiicient size for producing about W16" end movementof the metal when subjected to a temperautre change of about 100 F.either side of an ambient temperature, said flexibly adhesive materialbeing of sufficient thickness for accommodating said end movementwithout rupture upon diiferential dimensional variations produced bytemperature changes of about 100 F. either side of an ambienttemperature, said core being formed of plywood panels and said metalfacing being formed of aluminum of substantially no less than Ma"thickness.

3. A composite wear and impact resistant laminate comprising a coreformed of plural courses of wood panels superimposed in a staggeredarrangement, a metal facing overlying said core and formed of semi-rigidmetal panels, a lm of adhesive material integrally bonding superimposedwood courses one to the other, a second layer of adhesive materialintegrally bonding the underlying facing of the core to a supportstructure, and a third layer of exibly adhesive material integrallybonding the overlying face of the core to said metal facing, said coreand metal panels being substantially equal in length and said metalpanels being of sufficient size for producing about 9/16" end movementof the metal when subjected to a temperature change of about 100 F.either side of an ambient temperature, said flexibly adhesive materialbeing of sufficient thickness for accommodating said end movementwithout rupture said three adhesive materials being a cured-in-situsubstantially 100% reactive liquid synthetic rubber mastic, and saidsupport structure including, a metal layer, and a plurality of spacedsupports for said metal layer, said metal layer being bonded to saidunderlying facing of said core by said second layer of adhesive.

4. The composite of claim 3 wherein said third layer of adhesivematerial is a polysulde elastomer.

5. The composite of claim 4 wherein said second layer of adhesivematerial is formed of a polysuliide elastomer and a microsphere filler.

6. The composite of claim 3 wherein said third layer of exible materialis a polyurethane elastomer.

References Cited by the Examiner UNITED STATES PATENTS Lewis.

Elersten 52-411 Ensminger 156-278 Briefly 52-.404 l() Kohr 522-4622Sundelin et al 52-479 10 2,806,509 9/1957 Bozzaeco 52--309 2,925,8312/1960 Welty et al 52--309 OTHER REFERENCES Engineering Properties andApplication of Plastics: by Kinney 1957, pages 158 and 159, published byWiley and Sons, New York.

Plastics: July 1947; pages 21, 22 :and 24.

FRANK L. ABBOTT, Primary Examiner.

T. L. NACKENOFF, Examiner.

I. E. MURTAGH, Assistant Examiner.

1. A COMPOSITE WEAR AND IMPACT-RESISTANT LAMINATE COMPRISING A PLYWOODCORE FORMED OF PLURAL COURSES OF ADHESIVELY BONDED JPLYWOOD PANELS,SUPPERIMPOSED IN A STAGGERED ARRANGEMENT AND A COMPOSITE METAL ANDPLYWOOD FACING OVERLYING AND ADHESIVELY BONDED TO SAID CORER, SAIDCOMPOSITE FACING BEING FORMED OF LOOSELYINTERLOCKED METAL AND PLYWOODPANELS OF SUBSTANTIALLY EQUAL LENGTH AND OF A LENGTH CAPABLE OFPRODUCING APPROXIMATELY 3/16" END MOVEMENT OF THE METAL PANEL WHENSUBJECTED A TEMPERATURE CHANGE OF 100*F., A LAYER OF ADHESIVE MATERIALINTEGRALLY BONDING SAID PLYWOOD PANELS OF SAID FACING TO SAID PLYWOODCORE, AND ANOTHER LAYER OF FLEXIBLE ADHESIVE MATERIAL INTEGRALLY BONDINGTHE UNDER FACE OF EACH METAL PANEL TO ITS INTERLOCKED AND PLYWOOD PANEL.SAID LAYER OF FLEXIBLE ADHESIVE BEING OF SUFFICIENT THICKNESS